As can be observed from portable devices, electronic devices are now radically advancing toward acquiring a smaller, thinner, and more densely packaged body. Further, improvement in speed and functions of semiconductor elements has increased the number of their terminals, and this requires that a wiring board used for device mounting and semiconductor element mounting should be thinner, lighter, and more suited for dense packaging.
Typical wiring boards have been those that have through holes, such as built-up boards. However, such boards are thick and the through holes make the boards unsuitable for high-speed signal transmission.
Meanwhile, thin boards such as tape substrates have also been used. However, their manufacturing methods allow them to have only one or two wiring layers, and high elasticity of the tape substrates makes them be patterned less positionally accurately than built-up boards are patterned. Because of these factors, those thin boards cannot meet the current demands for highly dense packaging.
To solve these problems, Patent Literature 1 to Patent Literature 3 disclose a method of forming a wired structure on a prepared supporting substrate, and removing the supporting substrate after forming the wired structure, thereby forming a coreless substrate having no through hole.    Patent Literature 1: Unexamined Japanese Patent Application KOKAI Publication No. 2002-83893    Patent Literature 2: Unexamined Japanese Patent Application KOKAI Publication No. 2002-198462    Patent Literature 3: Unexamined Japanese Patent Application KOKAI Publication No. 2006-049819